There are few things that bring more discredit on the name of Christ than the divisions and quarrels among us. If we proclaim God's goodness and faithfulness, there are people who will not believe us even on that, because of how badly we disagree among ourselves. How do we reunite? Outside of Christ himself, there is no one person that all of us trust together to reach the right decision and settle the disputes and quarrels. Likewise there is no one group, and no one process of reasoning, that all of us together would trust.
On some level, the problem is a lack of leadership -- or, I should say, a lack of godly leadership. Over the years I have noticed something about the leaders compared to the people: in general the people in different groups of churches get along with each other fairly well, compared to the leaders. While there are some notable and noisy exceptions to that, it is more common for the members to get along than the leaders. Is it because the general people do not understand the differences, or because the leaders place less value on the common ground? I think there is some truth in each of those.
There are things that make different Christian groups incompatible; I wonder how many of those divisive things would remain, if there were no leaders advocating for them -- and no systems established to perpetuate those differences.
Paul's letter to Titus instructs him, as a church leader, to warn a divisive person once or twice -- and after that, if he persists in being divisive, to have nothing more to do with him. But if the leaders have become the divisive ones, who warns them?